Home Grown: A Green Roofs Primer

A rooftop garden typically includes six layers: the roof, a waterproof barrier, insulation, drainage/root barrier, substrate and vegetation. Click on the link above the image for more information on building a green roof for your home.

Illustration Courtesy Greensulate, greensulate.com

When Diana Cohen and her husband, Wes Slaymaker, decided they needed more space in their home in Madison, Wisconsin, they looked up rather than out. Hoping to enlarge their living area without increasing their ecological footprint, they decided to build an “addition” on top of their house by installing a green roof. After consulting a structural engineer, they resolved to build the 220-square-foot structure themselves. Several months of research, a few weekends of installation and $1,500 later, the Cohen-Slaymakers have a luscious outdoor living space where they can relax, grow food and watch birds, bees and butterflies at work.

You, too, could increase your family’s living and gardening space by installing a green roof. Although modern green roofs are a fairly new concept, the idea of rooftop gardens is ancient. The hanging gardens of Babylon, a series of terrace gardens that are one of the Seven Wonders of the World, were constructed in 600 B.C. in what is now Iraq. Sod houses dotted the American prairie during frontier settlement, and green roofs have been scattered across Scandinavia for centuries.

Modern green roofs—composed of vegetation and layers of growing medium such as shale and compost—were introduced in Germany in the late 1950s. They have since exploded in popularity throughout Europe and are on the rise in North America. Though green roofs are more common on commercial buildings, homeowners are also taking advantage of this living home improvement.

Raise the roof

There are plenty of reasons to get excited about a green roof. They can offer expanded outdoor space without requiring more land. They provide a fabulous place to grow food and host parties. And they attract birds and beneficial insects to your home.

Green roofs also help manage rainwater, diverting 50 to 90 percent of rooftop precipitation from the sewer system and returning it to the water cycle. Excess stormwater drains more gradually, reducing the sewer system’s peak flow volume. Green roofs aid efficiency by moderating temperature extremes. By providing modest thermal insulation, green roofs keep homes cooler in summer and a bit warmer in winter. High volumes of concrete and buildings create “heat islands” in many cities. Through heat absorption, green roofs help lower urban temperatures. Most roofs are exposed to ultraviolet rays, temperature extremes, hail and other elements. Green roofs buffer those elements and can double the lifespan of rooftop waterproofing.

For urban homeowners without yards, green roofs offer the opportunity to garden. “We’re seeing a lot of people moving toward rooftop agriculture, especially in areas where land space is difficult to come by,” says Ty Voyles, Green Roof Professional for DC Greenworks. “To use it to grow produce is a really inventive and ingenious way of using that space,” he says.

Root structure

Two green roof design approaches exist: built-up assemblies and modular or pre-assembled systems. A built-up system contains a series of individual layers: a root-repellant layer, a drainage layer (which allows water retention), a filter membrane, the growing medium and, finally, vegetation. Modular systems come with the drainage barrier, filter layer, growing medium and plants pre-assembled in molded plastic trays. The materials tend to be more expensive, but installation costs are lower, as these systems are simply laid in place and can be removed easily if necessary.

Green roofs do not use regular topsoil. The growing medium must be lightweight and resistant to erosion. A typical growing medium consists of expanded shale, mushroom compost and mineral components (ratios vary by location). Most homeowners choose to install 3 to 4 inches of medium. The thicker the growing medium, the deeper the root systems the roof can support, which allows for a greater variety of vegetation. Intensive green roofs, with at least 6 to 12 inches of media, can support shrubs and even trees, but often require daily irrigation and weigh at least 40 to 150 pounds per square foot.

Green roof vegetation typically consists of low-maintenance, drought-resistant plants such as sedums and alliums. Recommended plant species vary by climate and the growing conditions of the specific roof.

Best laid plans

If you want to install a green roof, first check local building codes, as well as financial incentives. Some counties and cities such as Portland, Oregon, and Chicago offer grants or tax credits for installing a green roof. Next, ensure that your roof is structurally sound and able to hold additional weight. Voyles recommends a dedicated load of 25 to 30 pounds per square foot for a standard 3- to 4-inch green roof. Consulting a roofing professional or structural engineer is the best way to ensure your roof satisfies this requirement.

It is also essential that the waterproofing roof membrane is in good condition to ensure your dwelling is watertight. A roofing professional can check for leaks. Depending on your roof material, you may choose to seal leaks or to add an additional water-proofing membrane. Some membranes, such as those made of heavy-duty PVC, can also serve as a root barrier, while asphalt or other more porous roofs require a separate root barrier layer.

Flat and pitched roofs both can be vegetated, though Voyles recommends creating a strategy for water and soil retention for roofs with a steep pitch. Roofs with steep pitches will create various “microclimates” on the same roof. Upper areas may tend to stay dry, while the bottom of slopes may remain damp nearly all the time. “The greater the pitch, the more the water travels downhill,” he says. “You combat that using different irrigation approaches or different plant selections that will work in the microclimates on the roof.”

Voyles estimates that a typical 3- to 4-inch-thick professionally installed residential green roof costs $12 to $18 per square foot for a low-slope roof. This includes materials, design and installation. The cost is greater if waterproofing, roof reinforcement or a structural engineer is needed.

Green at a Glance

Installing a rooftop garden requires these steps:

1. Consult a structural engineer to ensure your roof can support the additional weight of a green roof. Ask whether your waterproofing roof membrane will ensure your home is watertight. Is it heavy-duty enough to also act as a root barrier?

2. Choose whether to install a built-up system layer-by-layer or a pre-assembled modular system, and whether to install the roof yourself or hire a professional.

3. Install your system and growing medium.

4. Consult a local nursery to determine the best native, low-maintenance plants for your rooftop garden.

5. Plant your green roof and enjoy!

Lay It Out

A rooftop garden typically includes six layers.

• Roof: Nearly any roof, from flat to pitched, can host vegetation. Make sure your roof is in good condition before installing a living roof.

• Waterproof Barrier: Most roofs are covered with a waterproof barrier. Consult a professional to ensure yours is in good condition and will protect your roof from moisture. If not, you should repair or replace the waterproofing layer before installing a living roof.

• Insulation: An optional layer of insulation helps improve home efficiency and can be installed above or below the waterproof layer.

• Drainage/Root Barrier: The drainage layer is included in pre-assembled systems, or you can use a layer of gravel.

• Substrate: Most residential green roofs use 3 to 4 inches of “substrate,” or growing medium, consisting of compost, shale and other organic elements.